The present disclosure is related to a radiation image processing apparatus, a radiation image processing method, and a radiation image processing program that generates subtraction images from two dimensional images obtained by simple imaging and tomographic images obtained by tomosynthesis imaging.
Recently, tomosynthesis imaging has been proposed for radiation image obtaining apparatuses that employ radiation such as X rays and gamma rays. Tomosynthesis imaging obtains images by irradiating radiation onto a subject from a plurality of radiation source positions by moving a radiation source, and generates a tomographic image in which a desired cross sectional plane is emphasized, from a plurality of projection images obtained by the imaging operations, in order to observe an afflicted portion in greater detail. In tomosynthesis imaging, the radiation source may be moved parallel to a radiation detector or moved along a circular or elliptical trajectory according to the characteristics of an imaging apparatus or necessary tomographic images. The subject is imaged at a plurality of radiation source images to obtain a plurality of projection images. A tomographic image is generated by reconstructing the projection images by an inverse projection method such as the simple inverse projection method or a filtered inverse projection method. Structures that overlap in the depth direction in which cross sectional planes are arranged can be separated, by generating such tomographic images at a plurality of cross sectional planes within the subject. For this reason, it becomes possible to detect lesions, which had been difficult to detect within two dimensional images obtained by conventional simple imaging. Note that simple imaging is an imaging method in which radiation is irradiated onto a subject onetime, and a single two dimensional image, which is a transmission image of the subject, is obtained.
Meanwhile, there is a problem in tomosynthesis imaging that reconstructed tomographic images become blurred due to the influence of mechanical errors of an imaging apparatus, body movement of a subject caused by temporal differences among imaging operations at each of a plurality of radiation source positions, etc. If a tomographic image is blurred, it will become difficult for lesions such as fine calcifications, which are effective in early diagnosis of breast cancer, to be detected, in the case that the subject is a breast. For this reason, simple imaging is also performed in the case that tomosynthesis imaging is performed, to obtain both tomographic images and two dimensional images.
For this reason, radiation imaging apparatuses for imaging breasts (called mammography apparatuses) that perform both tomosynthesis imaging and simple imaging with breasts maintained in a compressed state have been proposed (refer to PCT Japanese Publication No. 2014-507250 and Japanese Unexamined Patent Publication No. 2012-166026).
In addition, subtraction processes are performed in mammography, in order to facilitate discrimination of lesions. Here, subtraction processes refer to a process in which an image corresponding to a difference in a plurality of radiation images imaged under different imaging conditions is obtained. Specifically, a subtraction process is administered for each corresponding pixel within the plurality of images, to emphasize or extract a specific portion of the subject within the radiation images, that is, to obtain a subtraction image.
There are basically two types of subtraction processes which are performed in mammography. One type of subtraction process is temporal subtraction. In temporal subtraction, an image, in which blood vessels of a breast are not emphasized, is subtracted from an image, in which the blood vessels of the breast are emphasized by imaging after injecting a contrast agent into the breast from a vein, to extract the blood vessel portions. The other type of subtraction process is energy subtraction. Energy subtraction utilizes the fact that contrast agents have different radiation absorption rates with respect to radiation having different energies. In energy subtraction, radiation having different energies are irradiated onto a breast after a contrast agent is injected, to obtain radiation images having different energies. The plurality of radiation images are weighted appropriately and differences among the radiation images are calculated, to extract the blood vessel portions of the breast. In addition, there are cases in which radiation is irradiated onto a breast at temporal intervals after a contrast agent is injected to obtain a plurality of radiation images. In such cases, the plurality of radiation images are weighted appropriately and differences among the radiation images are calculated, to ascertain the spread of the contrast agent through the blood vessel portions of the breast.
In many cases, breast cancer progresses by the blood vessel walls being weak and newly formed blood vessels that spread randomly toward the periphery increasing. The increased newly formed blood vessels increase transmissivity and are serpentine, different from normal blood vessels. Accordingly, the increased newly formed blood vessels can be extracted by employing subtraction images, thereby facilitating detection of breast cancer.
For this reason, a technique in which two radiation images to be subjected to a subtraction process are obtained to generate a subtraction image in addition to obtaining tomographic images by tomosynthesis imaging, and the subtraction image and the tomographic images are displayed has been proposed (refer to PCT Japanese Publication No. 2014-507250). In addition, a CE-DBT (Contrast Enhanced Digital Breast Tomosynthesis) technique that obtains radiation images to be subjected to subtraction processes at each radiation source position when performing tomosynthesis imaging has also been proposed (refer to Japanese Unexamined Patent Publication No. 2012-166026).
When obtaining radiation images, a scattered radiation removing grid (hereinafter, simply referred to as “grid”) is utilized when performing imaging, in order to prevent decreases in contrast due to the influence of scattered radiation which is generated within subjects. Meanwhile, tomosynthesis imaging performs imaging operations by irradiating radiation onto a subject from each of a plurality of radiation source positions. Therefore, the incident angle of radiation with respect to a radiation detector differs for each radiation source position. For this reason, if a grid is employed when performing imaging operations, vignetting occurs by the radiation being cut off by the grid depending on the radiation source position, and the amount of radiation that reaches the radiation detector will decrease. Accordingly, a grid is not employed when performing tomosynthesis imaging.
In addition, in the case that tomosynthesis imaging is performed, imaging operations are performed while moving a radiation source. Therefore, it is necessary to set the irradiation time of radiation onto a subject for each imaging operation to be as short as possible, to prevent blurring of projection images. Meanwhile, if the amount of irradiation time is set to be short, the amount of radiation which is irradiated onto the subject also decreases, resulting in the amount of radiation that reaches a radiation detector also decreasing. For this reason, high energy radiation that more readily passes through objects is employed in tomosynthesis imaging, in order to increase the amount of radiation that reaches a radiation detector with a minimal radiation dosage.